Ionic liquids with alkyl sulfates and functionalized alkyl sulfates are of considerable technical importance as halogen-free solvents, extractants and heat transfer media. While ionic liquids with methyl and ethyl sulfate anions are known to be readily available by alkylating an imidazole, pyridine, amine or phosphane with dimethyl sulfate or diethyl sulfate (P. Wasserscheid, C. Hilgers, EP-A-1 182 196), the preparation of all other representatives has been very complicated by the known method and frequently results in more or less contaminated products.
In the preparation of 1-butyl-3-methylimidazolium octyl sulfate, according to the prior art, the chloride salt of 1-butyl-3-methylimidazolium cation is reacted with sodium octyl sulfate with precipitation or extraction of sodium chloride (P. Wasserscheid, R. van Hal, A. Bösmann, Green Chem. 2002, 4(4), 400-404). In principle, this metathesis reaction is also suitable for the preparation of other ionic liquids with alkyl sulfate and functionalized alkyl sulfate ions, though the separation of the alkali metal halide requires tedious extraction and filtration steps. Further, even when working with absolute solvents, contaminants containing halide and/or alkali metal ions are found in the ionic liquid product.